Roller cone drill bits used to drill oil and gas wells often have a sealed lubricating system. Such drill bits generally incorporate a compensating mechanism to limit any pressure differential between lubricant sealed within the drill bit and well bore fluid adjacent to the drill bit. A common device is a flexible diaphragm separating the two fluids. The diaphragm responds to downhole conditions to maintain balanced pressure across the faces of fluid seals associated with the lubricating system. Flexible diaphragms also typically compensate for volumetric changes of lubricant which may occur in the form of leakage or through thermal expansion and contraction. Under certain conditions, lubricant expansion may exceed volumetric design limits of the associated lubricating system and generate an undesirable high pressure within the lubricating system. Damage to the fluid seals located to prevent the ingress of well bore fluid and escape of lubricant from the lubricating system is likely to occur if such high pressure is not satisfactorily vented or released.
Previous attempts with various styles of mechanical and/or elastomeric plunger type relief valves had limited success. Several drill bit manufacturers currently supply variations of two acceptable relief valve designs for achieving pressure relief. One system incorporates a disc spring which applies an axial sealing force to the periphery of the diaphragm. At some selected pressure differential, the spring force is overcome to relieve pressure in a manner similar to a pop-off type relief valve. The second system embodies a slit or puncture of some form within the compensating diaphragm itself.
Excess pressure is vented through the slit or puncture as it deforms when subjected to a selected pressure differential. This type of system may function as a one-way or two-way relief valve depending upon selected design. U.S. Pat. No. 3,847,234 entitled Pressure Relief Device for Drill Bit Lubrication System teaches the use of a separate two-way relief diaphragm, and associated seating components, threadably retained in a special vent passage formed in the bit.